In mammals the dorsal lateral geniculate nucleus (DLGN) is the principal structure receiving synaptic input from retinal ganglion cells of the eyes. In Golgi material from DLGN of cat, four classes of neurons can be defined according to cell size and dendritic morphology. The object of this study is to examine the fine structure of the synaptic organization of retinal input to identified cells of three of the four defined classes. Procedures will be employed for labeling and identifying individual cell types with electron dense material in the cat and for locating corresponding cell types in the monkey by anatomical position. The cells will be labeled by these methods: (1) diffuse staining with horseradish peroxidase (HRP): (2) deimpregnation and replacement of Golgi deposits with gold; (3) retrograde transport of HRP. Labeled cells and their synaptic relations will be examined in long series of consecutive thin sections with the electron microscope. Since correlated anatomical and physiological evidence suggests that each class of neuron has a specific function, the close comparison of the quantified synaptic relations and physiological properties of such cells should supply numerous insights into the function of DLGN and the nervous system in general.